Interpretive Summary: The rising concentration of carbon dioxide gas in the Earth’s atmosphere threatens to change the climate. Scientists around the world are measuring the exchange of carbon dioxide between the land and atmosphere to better understand this exchange, but they need to develop new techniques to partition this net exchange between photosynthetic uptake and ecosystem respiration release. This study examines the utility of a new partitioning technique using naturally occurring atmospheric carbonyl sulfide or OCS. We found that photosynthesis, estimated using the OCS technique, was similar to estimates derived from other common techniques. In the future, with similar OCS measurements at other sites, the technique proposed in this paper may provide an important independent estimate of photosynthesis and lead to a more accurate picture of global carbon dioxide cycling.

Technical Abstract:
Regional and continental scale studies of the seasonal dynamics of atmospheric carbonyl sulfide (OCS) mole fractions and leaf-level studies of plant OCS exchange have shown a close relationship with those for CO2. CO2 has sinks and sources within terrestrial ecosystems, but the primary terrestrial exchange for OCS is thought to be leaf uptake, suggesting potential for OCS uptake as a proxy for gross primary production (GPP). We examined the utility of OCS uptake as a GPP proxy in micrometeorological studies of biosphere-atmosphere CO2 exchange. Theoretical concepts from earlier OCS studies were combined to relate net ecosystem exchange (NEE) and vertical mole fraction gradients of CO2 and OCS to GPP. At the Harvard Forest AmeriFlux site, measured CO2 and OCS vertical gradients were correlated and were related to NEE and GPP, respectively. Estimates of GPP from OCS-based NEE partitioning were similar to GPP derived with established techniques, providing evidence that OCS uptake can potentially serve as a proxy for GPP. Measured vertical CO2 mole fraction gradients at five other AmeriFlux sites were used to project vertical OCS mole fraction gradients to provide indication of potential OCS signals at other sites where no OCS data were available. At the three forest sites, projected OCS gradients were similar in magnitude to those at Harvard Forest. At the two sites with short canopies (C4 grassland and soybean), projected OCS gradients were larger than those in forests, indicating greater potential for OCS uptake as a GPP proxy at these sites.